Metal powder including diffusion alloyed molybdenum

ABSTRACT

Metal powders for surface coating are disclosed. The metal powders comprise molybdenum diffusion alloyed to the base powder particles. Metal powders comprising a prealloyed powder and a molybdenum source are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a divisional application of application Ser.No. 10/270,728, filed Oct. 16, 2003, which is a Continuation-in-Part ofapplication Ser. No. 09/900,982, filed on Jul. 10, 2001; is a divisionalapplication of International Application No. PCT/SE02/00943, whichdesignates the United States of America and was filed on May 17, 2002;and claims priority to Swedish Application No. 0101776-3, filed on May18, 2001, the entire disclosure of each of which is hereby incorporatedby reference.

FIELD OF THE INVENTION

[0002] The present invention concerns thermal spray powders, theirproduction and use. Specifically, the invention concerns coating ofaluminum substrates with thermally sprayed metal powders.

BACKGROUND

[0003] Different methods for producing coatings on aluminum substratesare previously known. These methods are used in, e.g., aluminum engineblocks having cylinder liners which are formed by thermal spraying.

[0004] U.S. Pat. No. 2,588,422 discloses an aluminum engine block havingcylinder liners which are formed by thermal spraying. These liners arebuilt up in two layers on the untreated surface of the engine block, thetop layer being a hard slide layer such as steel about 1 mm in thicknessand the lower layer being a molybdous interlayer about 50 μm inthickness. The interlayer, containing at least 60% molybdenum, does notconstitute a slide layer, but is necessary in order to bind the hardslide layer to the aluminum block. Preferably, the interlayer is made upof pure molybdenum. The slide layer is a layer of hard metal, as forexample carbon steel, bronze or stainless steel, in which the steel maybe an alloy containing nickel, chromium, vanadium or molybdenum, forexample. In principle, this two-layer structure provides a good slidelayer, but the cost of the double coating is substantial.

[0005] In recent thermal spraying methods, the thermal spray powders aremade up by a mixture of powdered steel with powdered molybdenum, such asdescribed in the U.S. Pat. No. 6,095,107. The risk of segregation due todifferences in properties between the base steel powder and the powderof crushed molybdenum is however a problem which may result innon-uniform coatings. Another disadvantage is that comparatively largeamounts of molybdenum are required due to the segregation effect.

[0006] A main object of the present invention is to provide aninexpensive metal powder for thermal coating of substrates, especiallyfor aluminum.

[0007] Another object is to provide a powder, which does not segregateand wherein the amount of expensive molybdenum alloying metal can bereduced in comparison with currently used methods.

[0008] A further object is to provide a thermal powder, which has highdeposition efficiency and gives excellent coating quality.

[0009] Another object is to provide a thermal powder giving coatings ofsuitable porosity and oxide content and wherein the pores arepredominantly closed, isolated and have an advantageous range of porediameters.

SUMMARY

[0010] These objects are obtained by a metal powder comprising apre-alloyed iron base powder having particles of molybdenum, such asreduced molybdenum trioxide, diffusion alloyed to the particles of thebase powder.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] In the attached drawings:

[0012]FIG. 1 is a micrograph of a metal powder in which molybdenum isdiffusion alloyed to the surface of base powder particles.

[0013]FIG. 2 is a map showing the distribution of molybdenum in themetal powder shown in FIG. 1.

[0014]FIG. 3 is a map showing the distribution of iron in the metalpowder shown in FIG. 1.

[0015]FIG. 4 is a micrograph of a coating formed by applying the metalpowder shown in FIG. 1 on an aluminum substrate by plasma spraying.

DETAILED DESCRIPTION

[0016] The type and particle size of the iron base powder is selected inview of the desired properties of the final coating and the substrate.The base powders are preferably pre-alloyed with elements desired in thecoating. Also, a minor part of the molybdenum content may be included inthe pre-alloyed powder. Other elements, which may be included in thepre-alloyed base powder are C, Si, Mn, Cr, V and W. In the following,all % are in weight unless otherwise indicated. The pre-alloyed powdermay be prepared by atomization with water or gas. The particle sizes ofthe base powder are below 500 μm, preferably between 25 and 210 μm forPTA and less than 90 μm, preferably less than 65 μm, for high-velocityoxygen fuel (HVOF) or plasma spraying.

[0017] According to the present invention the base powder and thealloying powder, i.e., the source of the alloying element, which ispreferably molybdenum trioxide, are mixed according to the prescribedformulation and the mixture is heated to a temperature below the meltingpoint of the obtained mixture. The temperature should be sufficientlyhigh to ensure adequate diffusion of the alloying element into the ironbase powder in order to form a partially or diffusion alloyed powder. Onthe other hand, the temperature should be lower than the temperaturerequired for complete pre-alloying. Usually the temperature is between700° and 1000° C., preferably between 750° and 900° C., and thereduction is performed in a reducing atmosphere, e.g., hydrogen, for aperiod of 30 minutes to 2 hours for reduction of the molybdenumtrioxide, which is a preferred molybdenum source. As an alternativemolybdenum source, metallic molybdenum may be used.

[0018] Metal powders having molybdenum diffusion alloyed to base powderparticles are known. See, e.g., Japanese Patent Publication Nos.8-209202, 63-137102 and 3-264642. These known powders are, however, usedwithin the powder metallurgical industry for producing sintered productsof various shapes and sizes. Furthermore, and in contrast to the powdersaccording to the present invention, these known powders often have notonly molybdenum, but also copper and/or nickel diffusion alloyed to thebase powder particles. In these known powders the content of thediffusion alloyed molybdenum is usually low, whereas in the inventivepowder for thermal coating, the diffusion alloyed molybdenum ispreferably high in order to obtain efficient sliding. The mostinteresting results that have been obtained are with powders having acontent of diffusion alloyed molybdenum above about 4% by weight.Furthermore, only molybdenum is diffusion alloyed to the base powderparticles in the powder used for thermal spraying according to thepresent invention. The upper limit of this diffusion alloyed molybdenumis decided by how much molybdenum the base particles can carry, whichappears to be about 15% by weight, as described below.

[0019] The particle size of the final thermal sprayed powder isessentially the same as that of the pre-alloyed base powder, as themolybdenum particles, which are obtained when the molybdenum trioxide isreduced, are very small in comparison with the particles of the basepowder. The amount of the molybdenum that is diffusion alloyed to thebase powder should be at least 2% by weight of the total powdercomposition. Preferably, the amount of molybdenum should be between 2and 15, and most preferably between 3 and 10% by weight.

[0020] The different methods for applying the diffusion alloyed powderson the metal base substrate are spray or weld cladding processes, suchas flame spray, HVOF and plasma spray or PTA.

[0021] The invention is further illustrated by, but should not belimited to, the following preparation and example.

EXAMPLE

[0022] For the experiment on the new material for thermal coating basedon water atomized Fe-based (Fe-3Cr-0.5Mo)+5% Mo.

[0023] Used base materials and chemical analysis:

[0024] Water atomized iron powder (Fe-3Cr-0.5Mo)-71 μm Molybdenumtrioxide MoO₃ (average particle size 3-7 μm) Chemical Analysis(Fe—3Cr-0.5 Mo) % O-tot 1.22 C 0.48 Fe Base Ni 0.05 Mo 0.52 Mn 0.10 S0.01 P 0.01 Cr 2.95 Si <0.01 Sieve Analysis (Fe—3Cr-0.5 Mo) μm % 71-1060.1 63-71 0.8 53-63 4.7 45-53 23.4 36-45 23.1 20-36 33.3 −20 14.6

[0025] Procedure

[0026] 92.46% of the water atomized (Fe-3Cr-0.5Mo powder) and 7.54% ofMoO₃ were mixed together in a Lödige mixer and the annealing was carriedout as follows:

[0027] Temperature: 820° C.

[0028] Time: 60 min

[0029] Atmosphere: Reducing atmosphere (type H₂, N₂, CO₂ and mixtures ofthese gases)

[0030] After annealing the powder cake was crushed and sieved to aparticle size below 75 μm.

[0031] Sieve analysis and chemical composition (powder mix afterannealing): Chemical Analysis (Fe—3Cr-0.5 Mo) % O-tot 1.5 C 0.60 Fe BaseNi 0.05 Mo 5.57 Mn 0.10 S 0.01 P 0.01 Cr 2.75 Si <0.1 Sieve Analysis(Fe—3Cr-0.5 Mo) μm % 71-106 0.1 63-71 1.4 53-63 6.5 45-53 34.1 36-4520.4 20-36 30.8 −20 6.7

[0032] The obtained powder was used in a plasma spraying process forcoating an aluminum base substrate. An unexpectedly homogenous andexcellent coating was obtained with a minimum amount of molybdenum. FIG.1 is a micrograph of the powder showing molybdenum diffusion alloyed tothe surface of the base powder particles. FIG. 2 is a map showing thedistribution of molybdenum (in light spots) in the metal powder shown inFIG. 1. FIG. 3 is a map showing the distribution of iron in the metalpowder shown in FIG. 1. FIG. 4 shows the plasma sprayed coating appliedon the Al base substrate.

[0033] While the invention has been described in detail with referenceto preferred embodiments thereof, it will be apparent to those skilledin the art that various changes and modifications can be made, andequivalents employed, without departing from the scope of the appendedclaims.

What is claimed is:
 1. A metal powder for surface coating, the metalpowder consisting essentially of particles of a prealloyed powder andfrom about 4% to about 15% by weight of Mo diffusion alloyed to theparticles of the prealloyed powder, the prealloyed powder comprising Feand optionally at least one element selected from the group consistingof C, Si, Mn, Cr, V, Mo and W, the balance being inevitable impurities.2. The metal powder of claim 1, wherein the amount of molybdenumdiffusion alloyed to the particles of the prealloyed powder is fromabout 6% to about 10% by weight of the metal powder.
 3. The metal powderof claim 1, wherein the amount of molybdenum diffusion alloyed to theparticles of the prealloyed powder is from about 4% to about 10% byweight of the metal powder.
 4. The metal powder of claim 1, wherein theprealloyed powder is an atomized powder.
 5. A surface coating formed byapplying the metal powder according to claim 1 on a substrate by athermal spraying process.
 6. A surface coating formed by applying themetal powder according to claim 1 on a substrate by a weld claddingprocess.
 7. A metal powder for surface coating, the metal powdercomprising particles of a prealloyed powder and from about 4% to about15% by weight of Mo as the only element diffusion alloyed to theparticles of the prealloyed powder, the prealloyed powder comprising Feand optionally at least one element selected from the group consistingof C, Si, Mn, Cr, V, Mo and W, the balance being inevitable impurities.8. The metal powder of claim 7, wherein the amount of molybdenumdiffusion alloyed to the particles of the prealloyed powder is fromabout 6% to about 10% by weight of the metal powder.
 9. The metal powderof claim 7, wherein the amount of molybdenum diffusion alloyed to theparticles of the prealloyed powder is from about 4% to about 10% byweight of the metal powder.
 10. The metal powder of claim 7, wherein theprealloyed powder is an atomized powder.
 11. A surface coating formed byapplying the metal powder according to claim 7 on a substrate by athermal spraying process.
 12. A surface coating formed by applying themetal powder according to claim 7 on a substrate by a weld claddingprocess.
 13. A metal powder for surface coating, the metal powder beingproduced by heating a mixture consisting essentially of a prealloyedpowder and a molybdenum-containing powder, the metal powder comprisingfrom 4% to 15% by weight of Mo as the only element diffusion alloyed tothe particles of the prealloyed powder, the prealloyed powder comprisingFe and optionally at least one element selected from the groupconsisting of C, Si, Mn, Cr, V, Mo and W, the balance being inevitableimpurities.
 14. The metal powder of claim 13, wherein the mixture isheated at a temperature that is (i) sufficiently high to ensurediffusion of molybdenum into the prealloyed powder and (ii) lower than atemperature required for complete pre-alloying.
 15. The metal powder ofclaim 13, wherein the molybdenum-containing powder is MoO₃ or metallicmolybdenum powder, and the mixture is heated in a reducing atmosphere ata temperature between 750° C. and 900° C.
 16. The metal powder of claim13, wherein the amount of molybdenum diffusion alloyed to the basepowder particles is from about 6% to about 10% by weight of the metalpowder.
 17. The metal powder of claim 13, wherein the amount ofmolybdenum diffusion alloyed to the base powder particles is from about4% to about 10% by weight of the metal powder.
 18. The metal powder ofclaim 13, wherein the prealloyed powder is an atomized powder.
 19. Asurface coating formed by applying the metal powder according to claim13 on a substrate by a thermal spraying process.
 20. A surface coatingformed by applying the metal powder according to claim 13 on a substrateby a weld cladding process.